 /*******************************************************************************
  * Copyright (c) 2006 IBM Corporation and others.
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
  * which accompanies this distribution, and is available at
  * http://www.eclipse.org/legal/epl-v10.html
  *
  * Contributors:
  * IBM Corporation - initial API and implementation
  ******************************************************************************/

 package org.eclipse.ui.internal.expressions;

 import org.eclipse.core.expressions.EvaluationResult;
 import org.eclipse.core.expressions.Expression;
 import org.eclipse.core.expressions.ExpressionInfo;
 import org.eclipse.core.expressions.IEvaluationContext;
 import org.eclipse.ui.ISources;
 import org.eclipse.ui.IWorkbenchPart;

 /**
  * <p>
  * An expression that is bound to a particular part instance.
  * </p>
  * <p>
  * This class is not intended for use outside of the
  * <code>org.eclipse.ui.workbench</code> plug-in.
  * </p>
  *
  * @since 3.2
  */
 public final class ActivePartExpression extends Expression {

     /**
      * The seed for the hash code for all schemes.
      */
     private static final int HASH_INITIAL = ActivePartExpression.class
             .getName().hashCode();

     /**
      * The part that must be active for this expression to evaluate to
      * <code>true</code>. This value is never <code>null</code>.
      */
     private final IWorkbenchPart activePart;

     /**
      * Constructs a new instance of <code>ActivePartExpression</code>
      *
      * @param activePart
      * The part to match with the active part; may be
      * <code>null</code>
      */
     public ActivePartExpression(final IWorkbenchPart activePart) {
         if (activePart == null) {
             throw new NullPointerException ("The active part must not be null"); //$NON-NLS-1$
 }
         this.activePart = activePart;
     }

     public final void collectExpressionInfo(final ExpressionInfo info) {
         info.addVariableNameAccess(ISources.ACTIVE_PART_NAME);
     }

     protected final int computeHashCode() {
         return HASH_INITIAL * HASH_FACTOR + hashCode(activePart);
     }

     public final boolean equals(final Object object) {
         if (object instanceof ActivePartExpression) {
             final ActivePartExpression that = (ActivePartExpression) object;
             return equals(this.activePart, that.activePart);
         }

         return false;
     }

     public final EvaluationResult evaluate(final IEvaluationContext context) {
         final Object variable = context.getVariable(ISources.ACTIVE_PART_NAME);
         if (equals(activePart, variable)) {
             return EvaluationResult.TRUE;
         }
         return EvaluationResult.FALSE;
     }

     public final String toString() {
         final StringBuffer buffer = new StringBuffer ();
         buffer.append("ActivePartExpression("); //$NON-NLS-1$
 buffer.append(activePart);
         buffer.append(')');
         return buffer.toString();
     }
 }

